The present invention relates generally to sensor devices. More particularly, but not by way of limitation, the present invention relates to devices and methods for monitoring and communicating environmental conditions such as temperature, pressure, humidity, solar radiance, etc.
The monitoring of environmental conditions has become critical in many applications. For example, the monitoring of temperature has become critical in food storage devices, perishable-item transportation systems, environmental controls, biological product management (e.g., blood shipments), mechanical failure warning devices (e.g., engine overheating detectors and wing icing detectors) and other similar devices. Additionally, other fields require that conditions such as humidity, pressure, and solar radiance be monitored.
With regard to the monitoring of temperature, for example, known systems utilize thermocouples and/or silicon based temperature measurement devices. A typical temperature system using a thermocouple is shown in FIG. 1.
One of the significant problems of any electronic environmental sensor such as temperature sensors is with the calibration of these types of devices (these types of issues are also present with other types of environmental sensors such as pressure, humidity and the like as well). Further different types of sensors are needed, just as in the temperature fields with different ranges of temperatures. The calibration and re-calibration of these types of system using different types of sensors or even the same types of sensors requires extensive time and effort either during the manufacturing process or during use.
If a direct to digital sensing system is to be deployed a user needs to know that the sensors and the system can be calibrated or is calibrated. An analog to digital conversion can be done by conventional techniques, but the calibration of the output to the real world and within the ranges of the sensor need to be made with respect to the real world ranges to be monitored.
Before describing an exemplary thermocouple system and its limitations, however, a brief background of known thermocouple technology is necessary. Thermocouples are temperature measurement devices that operate according to the Seebeck effect in which a unique self-generated voltage is produced at a given temperature when two dissimilar metals are joined together. In an effort to maximize performance, numerous combinations of metals have been examined to determine their output voltage versus temperature range. Two of the more popular metal combinations have been characterized under conventional industry terminology as Type K and Type E thermocouples. Although the full-scale output voltage of all thermocouples falls within the millivolt range, Type E thermocouples have the highest output with almost 80 mV at 1800xc2x0 C.
To measure this relatively small output voltage, it is necessary to make connections to the wires forming the thermocouple. These connections form a second thermocouple (referred to as the cold junction) in series with the original thermocouple (referred to as the hot junction). To correct for any voltage output by the second thermocouple, the second thermocouple is often electronically corrected to zero degrees, i.e., the voltage is electronically corrected.
In the case of electronic correction, the temperature at the cold junction is measured, and the voltage that would be generated by the cold junction at that temperature is subtracted from the actual voltage reading. If the voltage versus temperature transfer function of the second thermocouple was highly linear, this subtraction would be all that was necessary to correct the reading. Unfortunately, the full-scale transfer function is usually fairly complex and requires several piece-wise approximations to maintain a specified accuracy.
Now referring to the typical thermocouple system 100 shown in FIG. 1, this version includes three thermocouples 102, 104, 106 for measuring temperature at three different locations. Each of these thermocouples 102, 104, 106 is connected by individual analog signal lines to specifically matched amplifiers 108, 110, 112 and each amplifier 108, 110, 112 is connected to a different input of a computer.
In operation, a thermocouple, such as thermocouple 102, generates a small voltage in response to a certain temperature. For a typical thermocouple, that voltage might be in the range of 1 mV at 25xc2x0 C. Because this voltage is so small, it is fed into an amplifier 108 that is powered by voltage source Vs. The amplified voltage level is then communicated (through an analog transmission line) to the computer 114, which can translate the amplified voltage level into an actual temperature reading.
Although thermocouple systems like the one shown in FIG. 1 are somewhat effective, they are often prohibitively expensive and often lack the necessary resolution and accuracy for widespread use. As mentioned above, present thermocouple systems require a matched amplifier for each thermocouple. These amplifiers introduce added cost and added inaccuracies. Additionally, the analog output of the typical thermocouple, which is only in the millivolt range, is subject to interference by noise. To limit the impact of noise, the amplifier and the computer should generally be placed close to the thermocouple, thereby significantly limiting the placement of the thermocouple. Moreover, in certain embodiments, electromagnetic interference (EMI) shielding is required to limit the impact of noise. Of course, this shielding can introduce non-trivial additional costs.
Another problem with present thermocouple systems is their lack of expandability and adaptability. For example, the number of thermocouples that a system can use is generally limited to the number of input pins for the computerxe2x80x94although some embodiments use multiplexers, and other complicated systems, to expand the number of thermocouples that can be connected to a computer. Additionally, each thermocouple, which only transmits analog voltage signals, should be placed on its own line rather than placing multiple thermocouples on each line. Thus, wiring the system shown in FIG. 1 often requires duplicative wiring and the associated additional costs.
Accordingly, present thermocouple systems suffer from significant deficiencies that limit the use of an otherwise beneficial technology. Although some of these deficencies are alleviated, but not eliminated, by silicon-based measurement devices, these devices also suffer from drawbacks such as limited temperature range. Thus, a device and method are needed that overcome these and other drawbacks in the present technology. In particular, but not by way of limitation, a system and method are need that efficiently, effectively and accurately measure temperature and other environmental conditions. Such a device and method could result in significant savings in both time and money for many industries and, additionally, could result in the spread of monitoring devices to industries that once shyed away from such devices because of excessive costs.
To remedy the deficiencies of existing systems and methods, the present invention provides a method and apparatus to monitor environmental conditions.
One of the various embodiments of the present invention includes: a thermocouple configured to generate a voltage indicative of how hot a junction temperature is; a memory device configured to store a unique device ID and to store data; a logic unit connected to the thermocouple and the memory device; an I/O interface connected to the logic unit, the I/O interface configured to communicate with a computer system; and an internal temperature sensor connected to the logic unit, the internal temperature sensor configured to determine a cold junction temperature. In this embodiment, the logic unit is configured to use the voltage generated by the thermocouple and the cold junction temperature to produce a digital indication of the hot junction temperature.
Further this type of arrangement, is only illustrative of the type of sensor that can be used in this type of system. The calibration of the sensor, be it a temperature device such as a thermocouple, silicon temperature sensor or the like, or a humidity device as will be discussed below or any other type of environmental sensor can be individually identified and addressed with a minimal number of connections and more importantly a calibration adjustment can be tied to each device. This tied calibration can be as complex as needed, using multiple coefficient polynomials or a simple adjustment using an offset or almost anywhere in between. The ability of any sensor to be tied to an address or to have a unique identity allows the reading device depending on the level of accuracy needed or desired to tie a calibration to a given sensor. Further, the unique ID address can be used as a URL extension or part of a URL address to download calibration data from or over, for example the Internet or an Intranet.